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On a sunny summer day it’s common to see lines of turtles sunning themselves on logs. That’s because reptiles are cold-blooded, also known as ectotherms. They produce very little body heat and instead absorb warmth from the environment to support their activity and digestion. This lifestyle works well during the hotter parts of the year, but things get difficult when temperatures begin to drop. Starting in late fall, reptiles begin to prepare for dormancy.

Since reptiles don’t make their own heat, it’s important for them to find shelter that will keep them from freezing over the winter. These shelters are called hibernacula (the singular form is hibernaculum). Some, like box turtles, dig burrows and make their own hibernacula. Other reptiles, like garter snakes, have to find existing structures. Snakes in warmer climates can make do with a rotting tree stump or an abandoned mammal burrow. Snakes from colder climates in the north have to find more elaborate hibernacula to stay below the frost line.

Photo by Tim Walsh

Up to half of garter snakes will die during the winter, even among those who are sheltered.

Underground caves can be attractive refuges for garter snakes. Favorite caves can hold thousands of snakes at once, all overwintering in a massive pile. More adventurous snakes may choose a basement as their hibernaculum of choice. While this may be startling to home owners, garter snakes are generally harmless. Investigating snakes should be gently removed, or relocated to a wildlife rehabilitation center.

Aquatic turtles have a different strategy. They sink to the bottom of the ponds where they live and overwinter there. The cold slows their metabolism to a crawl. Turtles can get oxygen through their skin or their cloaca (a turtle’s anus); so many turtles can stay underwater for days at a time in winter when the water is very cold.

What these reptiles go through each winter may sound a lot like mammalian hibernation, but in reality, it is rather different. When hibernating, mammals fall into a deep sleep. Dormant reptiles are not asleep; rather they are in a state of suspended activity. A particularly warm winter day may in fact draw reptiles outside again, before retreating once more into their hibernaculum.

Photo by Tim Walsh

Alligators can dig impressive burrows using their snout and their tails. They shelter in these burrows when the weather is cold enough for dormancy.

Yearly dormancy is something that reptile owners have to take in mind as well. Some keep their reptiles warm and active throughout winter while others simulate dormancy conditions like the reptile would encounter in the wild.